The present invention relates to data processing apparatuses which include preprogrammed functions and at least one user accessible memory. This combination of functional capability is most often found in the so called "scientific" calculators. These calculators generally include a wide variety of functions in addition to the four basic arithmetic operational functions of addition, subtraction, multiplication and division. These machines typically include a keyboard for entering information and operational commands into the machine and a visual display for communicating an indication of the results to the operator. These machines often include one or more user accessible memories. A user accessible memory is a data memory which the operator can selectively address for both storing and recalling data. Typically only a single keystroke is necessary for either storing or recalling data in the case in which a single user accessable memory is provided and a two keystroke sequence including some means to designate the particular memory addressed is employed in a machine which uses a plurality of user accessible memories.
A problem exists with this type of calculator organization. Often it is desirable to complete an operation on a displayed value and store that value in memory or to combine a displayed value with a stored value and to store the result in the same memory. This kind of function is useful when it is desirable to compute and store or compute, store and accumulate intermediate products of a lengthy or complicated mathematical expression.
It is known in the prior art to provide a dedicated input key for combining the completion of an operational command and storage within a user accessable memory. Thus previous calculators have included M+ keys for adding the displayed value into a user accessible memory or M- keys for subtracting the displayed value from the user accessible memory. Some machines go one step further by providing a SUM/PROD key which when used in conjunction with a second function or shift key and an inverse operation key is employed to provide combined operations and memory entry for each of the four basic arithmetic operations. Even with the use of a single key in conjunction with a second function key and an inverse key it is necessary to provide a particular dedicated key or key sequence for each of the operations to be combined with the memory entry. In addition it is not the usual practice to employ such combined operation and memory entry keys for single operand functions such as the trigonometric functions of sine, cosine and tangent. Therefore, this solution to the problem is less than completely satisfactory in many instances because it requires use of an additional key or keystroke sequence for each operation to be combined with memory entry thereby cutting down the number of available keys or keystroke sequences for other operations. This requirement for additional keys or keystroke sequences becomes a problem as the calculators themselves are being assembled in smaller and smaller packages with less and less space for any required entry keys.